Method for detecting multipath signals

ABSTRACT

The invention relates to a method for detecting multipath signals comprising the following steps: a total impulse response ( 10 ), composed essentially of multipath signals, is cross-correlated with a reference signal and a maximum ( 12 ) of the result of said cross-correlation is determined and saved.

[0001] The invention relates to a method for detection of multipathsignals, in particular for operation of a RAKE receiver, as claimed inpatent claim 1, and to a RAKE receiver as claimed in theprecharacterizing clause of patent claim 5.

[0002] Signals in general propagate over two or more paths in one mobileradio channel (multipath propagation). Each individual received signalhas a different propagation time, amplitude and Doppler shift,corresponding to its transmission path. The various received signals aresuperimposed at the antenna of a receiver either constructively ordestructively, and thus making it harder to detect the symbolstransmitted by means of the signals.

[0003] In particular, multipath propagation of a radio signal such asthis results in channel impulse responses over an extended time, whichlead to a delay spread and thus to a shifted time reference between theindividual symbols in the transmission signal. In mobile radio systemswhich operate using the Wideband Code Division Multiplex (W-CDMA)method, this means that special receivers (RAKE receivers) are requiredfor processing the multipath signals.

[0004] A RAKE receiver such as this allows a considerable gain as aresult of the use of multipath signals which arrive with differentpropagation time delays at a receiving antenna of a mobile radio. Thisis done by the signal coming from the antenna being processed in thereceiver in two or more paths, the so-called “fingers” of the RAKEreceiver. Each of these fingers is set, in each case with an optimizedphase angle of the pseudo-noise sequences which have been used forcoding of a signal to be transmitted via the mobile radio channel, to ineach case one component of the received multipath signals.

[0005] In an IS-95 mobile radio system, a RAKE receiver in a mobilestation has at least three such fingers, and a base station has at leastfour such fingers. Furthermore, both the mobile station and the basestation each have at least one “search finger”, which continuously looksfor stronger multipath signals. As soon as the search finger finds astronger multipath signal, the finger on the previously weakestmultipath signal is optimally set to the new, stronger multipath signal.Typically, this makes it possible to demodulate up to three of thestrongest multipath signals in the mobile station and up to four of thestrongest multipath signals in the base station, with a time delay of atleast 0.8 to 1 μs, and to combine them by means of maximum ratiocombining.

[0006] In this case, it is particularly important to detect the timereference between the individual multipath signals exactly, that is tosay to define the optimum “finger position”. A specially tuned filter, aso-called matched filter, thus uses a sequence or a pilot sequence whichis known in the receiver to produce the overall impulse response orchannel impulse response of a received signal, on the basis of which theposition of the individual fingers can be determined. However, theoverall impulse response does not always allow detection of all therelevant finger positions. Particularly when adjacent fingers differ,for example, only by a very short delay time of less than the chipduration, reliable detection of the position and of the number of thefingers is difficult.

[0007] Known methods for detection of the finger positions are, however,highly computation intensive since they have a complexity of O(N), whereN is the length of the overall impulse response.

[0008] The object of the present invention is thus to propose a methodfor detection of multipath signals and a corresponding RAKE receiverwhose data processing is less complex than the known detection methods,thus allowing fundamentally shorter adjustment times and a reducedprocessor capacity requirement.

[0009] This object is achieved by a method having the features of patentclaim 1 and by a RAKE receiver having the features of patent claim 5.Preferred refinements of the method can be found in the dependent patentclaims.

[0010] The idea on which the invention is based is to use the overallimpulse response to determine peaks or maxima successively, by means ofcross-correlation. Once a first maximum has been determined, the overallimpulse response is reduced by a reference signal, which wascross-correlated with the overall impulse response in order to determinethe first maximum. This means that maxima which indicate the possibleposition of fingers are determined step-by-step from the overall impulseresponse by means of the reference signal.

[0011] The method aspect of the invention accordingly relates to amethod for detection of multipath signals, in particular for operationof a RAKE receiver, having the following steps:

[0012] a) cross-correlation of an overall impulse response with areference signal,

[0013] b) determination and storage of a maximum of the result of thecross-correlation and of a proportionality factor α,

[0014] c) (if appropriate, repeated) reduction of the overall impulseresponse by the reference signal scaled by α,

[0015] d) carrying out steps a) to c) using the overall impulse responsereduced by the scaled reference signal until a predetermined number ofmaxima have been obtained, or until the scaling factor α falls below apredetermined value.

[0016] This method allows signals contained in the overall impulseresponse to be identified even when they have only very shortpropagation time differences, that is to say they are very close to oneanother in time. Normally, short propagation time differences such asthese between two signals mean that a pulse which is transmitted by thesignals arrives at a receiver extended in time. However, this is highlydisadvantageous for detection of pulses that are close to one another,for example in the case of chips in a signal transmitted by means ofCDMA. Furthermore, the method according to the invention involvesconsiderably fewer computation steps than the methods which are knownfrom the prior art. While the first two iteration steps fordetermination or selection of the first two maxima have a complexity ofO(N), all the further iteration steps now have only a complexity ofO(E²), where E is the width of the reference signal and N is the lengthof the overall impulse response.

[0017] The method is preferably used in a RAKE receiver. As alreadymentioned, RAKE receivers are used in particular in code-divisionmultiplex mobile radio systems. In principle, a RAKE receiver uses thepropagation time differences between multipath signals by having two ormore paths or fingers which are each set to the phase angle of onesignal from the multipath signals. The signals which are optimized inthe individual paths or fingers of the RAKE receiver are then combinedto form an overall received signal. The method according to theinvention allows the operation of a RAKE receiver to be improvedconsiderably, in particular with regard to an overall impulse responsefor individual signal components which have only very short propagationtime differences (approximately ¼ of a chip).

[0018] The reference signal which is used for the method is preferably afinger pattern which corresponds to a typical mobile radio channel. Inthis case, the expression a typical mobile radio channel means a mobileradio channel which has limited propagation time differences resultingfrom multipath propagation of transmitted signals, and which representsa good approximation of a large number of actual mobile radio channels.

[0019] The reference signal is preferably scaled such that a scaledreference signal essentially matches the cross-correlation maximumdetermined in step b) of the method. This considerably improves theefficiency of step c) of the method.

[0020] In one preferred embodiment of the invention, multiply andaccumulate operations are essentially carried out while carrying outsteps a) to d). These operations considerably improve the efficiency ofthe method in comparison to the methods which are known from the priorart, since a multiply and accumulate operation can be carried out withconsiderably less computation complexity than the operations ofsquaring, addition, subtraction and multiplication that are required forknown methods.

[0021] Finally, the invention relates to a receiver for carrying out themethod, which has two or more paths or fingers for processing ofreceived multipath signals. According to the invention, the receiver hasa cross-correlator for cross-correlation of an overall impulse responsewith a reference signal, a memory for the overall impulse response, anda scalar and subtractor in order to reduce the overall impulse responseby the reference signal.

[0022] The cross-correlator, the memory and the scalar and subtractorare preferably in the form of program routines in a program for a signalprocessor for processing the overall impulse response, or are in theform of programmable or hard-wired logic, in particular in an ASIC. Asignal processor has special functions and/or commands for signalprocessing and can thus carry out the abovementioned program routinesvery efficiently. Furthermore, a signal processor such as this can beoperated at a very high clock frequency, in order to make it possible toprocess the high data rates that occur. Implementation in the form oflogic has the advantage that this is very fast and can thus process evenhigher data rates.

[0023] Further advantages and application options of the invention willbecome evident from the following description of an exemplary embodimentin conjunction with the drawings, in which:

[0024]FIG. 1 uses signal profiles to show the processing of an overallimpulse response of received multipath signals, on the basis of themethod according to the invention,

[0025]FIG. 2 shows an example of the profile of a channel impulseresponse or overall impulse response of a mobile radio channel,

[0026]FIG. 3 shows the profile of a virtual channel impulse response oroverall impulse response of the mobile radio channel, whose actualoverall impulse response is illustrated in FIG. 2,

[0027]FIG. 4 shows a search pattern which is used as a reference signal,

[0028]FIG. 5 shows a further example of the profile of an overallimpulse response for a mobile radio channel, which essentially has threeindividual pulses and a noise component,

[0029]FIG. 6 shows a flowchart of one exemplary embodiment of the methodaccording to the invention, and

[0030]FIG. 7 shows a block diagram of one embodiment of the receiveraccording to the invention.

[0031] The basic procedure for the method according to the inventionwill be explained briefly in the following text using an exemplaryembodiment and with reference to FIG. 1. Examples of the profiles of theoverall impulse response and the search pattern and/or reference signalwill then be explained with reference to FIGS. 2, 3, 4 and 5.

[0032] The diagram A in FIG. 1 shows the overall impulse response 10 ofreceived multipath signals. The overall impulse response 10 essentiallyhas three maxima 12, 14 and 16, which occur at different receptiontimes. Using the method according to the invention, the overall impulseresponse 10 is cross-correlated with a reference signal (which is notillustrated). The result of the cross-correlation and the determinationof the maximum and of the scaling factor result in the signal which isillustrated in the diagram B, which essentially has one maximum 18,which corresponds to the maximum 14 at the time τ_(max1) in the diagramA, likewise at the time τ_(max1).

[0033] In the next step, the overall impulse response 10 as illustratedin the diagram A is reduced by the scaled reference signal. The resultof this subtraction is illustrated in the diagram C in FIG. 1. Thesignal 20 which is illustrated in the diagram C now has essentially twomaxima 22 and 24. The maximum 22 at the time τ_(max2) will be determinedfrom the signal 20 in a next step. The determination process is onceagain carried out by means of the cross-correlation process based on themethod according to the invention. The result of the cross-correlationprocess is illustrated in the diagram D. This has a maximum 26 at thetime τ_(max2), which corresponds to a second signal component in theoverall impulse response.

[0034] The signal 20 which is illustrated in diagram C is once againreduced by the scaled reference signal, thus resulting in the signal 28as illustrated in the diagram E, which essentially has only one maximum30 at the time τ_(max3), this maximum corresponds to a third signalcomponent in the overall impulse response 10. Finally, aftercross-correlation of the signal 28 as illustrated in the diagram E bymeans of the reference signal and subsequent determination, the maximum32 which is illustrated in the diagram F, is obtained at the timeτ_(max 3).

[0035] In the next step, the signal 28 as illustrated in the diagram Eis once again reduced by the scaled reference signal, thus resulting inthe signal 34 as illustrated in the diagram G. This signal 34 nowessentially comprises only noise components, so that the methodaccording to the invention for detection of multipath signals ends atthis point. Specifically, this means that the maxima 18, 26 and 32 whichare illustrated in the diagrams B, D and F have been extracted on thebasis of different propagation paths and propagation times in the mobileradio channel.

[0036]FIG. 2 shows an example of the profile of an overall impulseresponse 11 after passing through a matched filter in a RAKE receiver.The sample value i is plotted on the abscise, corresponding to aspecific reception time. The overall impulse response comprises signalcomponents from the various propagation paths in one mobile radiochannel. By way of example, FIG. 5 shows how an overall impulse response13 such as this is formed from superimposition of two or more individualpulses 15, 17, 19 and a noise component 21. The upper diagram in FIG. 5shows the noise component 21 with “noise”. Furthermore, the figure showsthree individual pulses 15, 17, 19 with finger 1, finger 2 and finger 3.The individual pulses each correspond to one propagation path andtypically arrive at different times in a receiver. The overall impulseresponse 13, which is composed of the noise component 21 “noise” and thethree individual pulses 15, 17, 19 finger 1 to finger 3, is shown in thelower diagram. The object of the receiver is now to use this overallimpulse response to determine start times of a transmitted data block.These start points can be used for reception. The start pointscorrespond to finger positions, that is to say in principle to thereception times of the individual fingers. A simple maximum search fordetermination of the finger positions in the overall impulse responseis, however, highly inaccurate and leads to poor reception. In acorresponding manner to the procedure described with reference to FIG.1, the overall impulse response is searched for a pattern whichcorresponds to the impulse response of a single propagation path in themobile radio channel. As soon as this pattern has been found, it issubtracted from the overall impulse response. This procedure is repeateduntil no more significant maxima occur in the continually reducedoverall impulse response.

[0037]FIG. 3 shows an example of the profile of a virtual overallimpulse response. This virtual overall impulse response is composed of asearch pattern or reference signal, whose magnitude can be scaled, andof an additional component γ. The search pattern is scaled using ascaling factor α. The component γ corresponds to a noise component.

[0038]FIG. 4 shows the search pattern contained in the virtual overallimpulse response in FIG. 3. There is a main maximum in the searchpattern at υ_(min), and a secondary maximum at υ_(max).

[0039] The flowchart which is illustrated in FIG. 6 shows the stepswhich are used according to the invention to extract the individualfinger positions from an overall impulse response. In a first step S1,the impulse response received by a receiver from one mobile radiochannel is subjected to matched filtering. A finger position counter isset to zero. In a second step S2, the overall impulse response is thencross-correlated with a reference function. The result of thecross-correlation is processed in a third step S3, which is used to finda maximum of the result of the cross-correlation and a proportionalityfactor α. This is followed by a question step A1, in which a check iscarried out to determine whether the maximum is a finger. If it is foundthat the maximum is not a finger, the method is ended, since the“residual” overall impulse response now contains only noise components.Otherwise, the maximum is stored in a step S4 as a defined fingerposition, and the overall impulse response is reduced by the searchpattern or reference signal that has been scaled using theproportionality factor α. The finger position counter is alsoimplemented. The next question step A2 checks whether the value of thefinger position counter has already exceeded a predetermined value. Thepredetermined value corresponds to a predetermined number of maxima orfinger positions. This is because the method can be terminated after aspecific number of maxima or finger positions. Typically, this number ofmaxima or finger positions corresponds to the “fingers” of a RAKEreceiver. However, if the predetermined value has not yet been reached,the method is continued with the step S2.

[0040] The way in which the search pattern is identified in the overallimpulse response will be explained briefly in the following text, usingthe least error squares method. Specifically, this means that the searchpattern in the overall impulse response is in fact identified where thesquare of the error between the overall impulse response and a virtualoverall impulse response (which has only one propagation path) is aminimum. This is essentially obtained by the cross-correlation. In otherwords, this means that there is a finger position at the point at whichthe cross-correlation of the overall impulse response with the searchpattern or the reference signal is a maximum. For this case, theproportionality factor α has particular importance. This is because theoverall impulse response is reduced by the search pattern or referencesignal that has been scaled by α.

[0041]FIG. 7 shows the essential means in a receiver R for carrying outthe method according to the invention.

[0042] In this sense, the receiver R comprises a cross-correlator 40, amatched filter 42 and a subtractor and scalar 44. A multiplexer 46 isalso provided, whose output signal is supplied to an impulse responsememory.

[0043] A data signal which has been transmitted via a mobile radiochannel is supplied to the matched filter 42 and the receiver R. Thematched filter 42 is used for equalization of the received data signal.At its output signal, the matched filter 42 produces the overall impulseresponse 54 of the mobile radio channel. The overall impulse response 54is supplied to a first input of a multiplexer 46. The overall impulseresponse 54 can be supplied via the multiplexer 46 to an impulseresponse memory 48 which is used for temporary storage of the samplevalues of the overall impulse response.

[0044] The output signal from the impulse response memory 48 is suppliedto the cross-correlator 40, and to the subtractor and scalar 44. Thecross-correlator 40 cross-correlates the overall impulse response, asstored in the impulse response memory 48, with a reference signal 50that has been supplied. The output signal from the cross-correlator 40is a finger position signal 56 and is supplied to the subtractor andscalar 44, to which the reference signal 50 can also be supplied, via aswitch. First of all, the subtractor and scalar 44 scales the referencesignal 50 that has been supplied via the switch, corresponding to thefinger position signal 56. It then reduces the supplied overall impulseresponse, as temporally stored in the impulse response memory 48, by thescaled reference signal 50. The subtraction output signal 52 is suppliedas a second input signal to the multiplexer 46.

[0045] The RAKE receiver R operates as follows:

[0046] A signal which is received via an antenna (which is notillustrated) is subjected to the conventional signal preprocessing, andis supplied to the matched filter 42. The output signal from the matchedfilter 42 is the overall impulse response 54, and is temporally storeddirectly in the impulse response memory 48, via the multiplexer. To bemore precise, the impulse response memory 48 stores sample values of theoverall impulse response 54. The overall impulse response which has beentemporally stored in the impulse response memory 48 is processed via thecorrelator 40 and the subtractor and scalar 44 using the methodaccording to the invention.

[0047] After processing, the multiplexer 46 is switched to the secondinput, that is to say the subtraction output signal 52 is temporallystored in the impulse response memory 48 via the multiplexer 46. Theoverall impulse response which has been temporally stored in the impulseresponse memory 48 is now processed via a controller (which is notillustrated) using the method according to the invention until eitherthe finger position signal 56 does not indicate any more new fingerpositions, or a predetermined number of finger positions have alreadybeen determined. This may be done, for example, simply by counting thefinger positions which are signaled by the finger position signal 56.

[0048] List of reference symbols

[0049]10, 11, 13 Overall impulse response

[0050]12, 14, 16, 22, 24, 30 Maximum

[0051]15, 17, 19 Individual pulses

[0052]18, 26, 32 Detected maximum

[0053]20, 28 Multipath signals reduced by the reference signal

[0054]21 Noise component

[0055]34 Noise signals

[0056]40 Cross-correlator

[0057]42 Matched filter

[0058]44 Subtractor and scalar

[0059]46 Multiplexer/demultiplexer

[0060]48 Impulse response memory

[0061]50 Reference signal

[0062]52 Subtraction output signal

[0063]54 Overall impulse response

[0064]56 Finger position signal

[0065] R RAKE receiver

1. A method for detection of multipath signals (10) which are receivedvia a mobile radio channel, in particular for operation of a RAKEreceiver having the following steps: a) cross-correlation of an overallimpulse response (10) with a reference signal, b) determination andstorage of a maximum (18) of the result of the cross-correlation and ofa proportionality factor α, c) reduction of the overall impulse response(10) by the reference signal scaled by α, d) carrying out steps a) to c)using the overall impulse response (10, 20, 28) reduced by the scaledreference signal until a predetermined number of maxima (18, 26, 32)have been obtained, or until the scaling factor α falls below apredetermined value.
 2. The method as claimed in claim 1, characterizedin that the reference signal is a window-limited impulse response of onepath of the mobile radio channel.
 3. The method as claimed in claim 1 or2, characterized in that the reference signal is scaled such that thescaled reference signal essentially matches the cross-correlationmaximum determined in step b).
 4. The method as claimed in one of thepreceding claims, characterized in that multiply and accumulateoperations are essentially carried out while carrying out steps a) tod).
 5. A RAKE receiver for carrying out the method as claimed in one ofthe preceding claims, which has two or more paths or fingers forprocessing received multipath signals (10), characterized in that across-correlator (40) is provided for cross-correlation of the overallimpulse response (10) with a reference signal (50), a memory (48) isprovided for the overall impulse response, and a subtractor and scalar(44) are provided in order to decrease the overall impulse response bythe scaled reference signal.
 6. The RAKE receiver as claimed in claim 5,characterized in that control logic is also provided in order to controlthe cross-correlator (40) and the subtractor and scalar (44).
 7. TheRAKE receiver as claimed in claim 5 or 6, characterized in that thecross-correlator (40), the memory (48) and the subtractor and scalar(44) are in the form of program routines in a program for a signalprocessor for processing multipath signals, or are in the form ofprogrammable or hard-wired logic, in particular in an ASIC.